1. Field of the Invention
The present invention relates to a semiconductor laser device, and more specifically, to a semiconductor laser device adapted for use as a pumping light source for optical fiber amplifier, such as a laser device of the gain-waveguide type that oscillates with a wavelength of, e.g., 0.98 xcexcm and requires high optical output of hundreds of mW, and capable of ensuring a linear current-optical output characteristic even during high-current operation.
2. Prior Art
In a semiconductor laser device that has an active layer (quantum region) formed of a quantum well structure, injected carriers are quantized toward quantum wells, and the state density of carrier energy is stepped. Accordingly, the gain coefficient suddenly rises in response to driving current, so that a laser beam can be oscillated even with use of a low threshold current density. The semiconductor laser device of this type delivers higher optical output than a semiconductor laser device that includes an active layer of a bulk semiconductor, so that it is being studied for practical use as a pumping light source for optical fiber amplifier.
The following semiconductor laser device that oscillates with a wavelength of 0.98 xcexcm was an object of investigation by the inventors as a pumping light source for optical fiber amplifier. This device will now be described with reference to the accompanying drawings. FIG. 1 is a side view showing the semiconductor laser device, and FIG. 2 is a sectional view taken along line IIxe2x80x94II of FIG. 1.
The device has a layer structure of a semiconductor material, including a lower clad layer 2 of n-AlGaAs, an active layer 3 of a quantum-well structure made of InGaAs and GaAs, an upper clad layer 4 of p-AlGaAs, and a cap layer 5 of p-GaAs, which are stacked in layers on an n-GaAs substrate 1. A part of the upper clad layer 4 and the cap layer 5 form a mesa structure, and a passivation film 6 of SiN is formed on the lateral of the mesa structure. Further, an upper electrode 7 of Ti/Pt/Au is formed on the cap layer 5 and the passivation film 6, and a lower electrode 8 of AuGe/Ni/Au is formed on the back surface of the substrate 1.
The device A is manufactured in the following manner. The aforesaid layer structure is formed on the n-GaAs substrate by, for example, the MOCVD method, and the upper and lower electrodes are formed on the upper and lower surfaces, respectively, of the layer structure. Thereafter, the resulting structure is cleft with a given cavity length L, a low-reflection film 9 of, e.g., SiN is formed on one end face (front facet) S1 of the structure, and a high-reflection film 10 of, e.g., SiO2/Si is formed on the other end face (rear facet) S2.
In the case of the device A having this mesa structure, it is believed that high optical output can be effectively obtained by increasing the cavity length L. This is because if the cavity length L increases, the influence of heat can be lessened, so that high-optical output can be expected. If the cavity length is too long, however, the differential quantum efficiency of the device A lowers, so that higher current is required for high-optical output operation. Normally, therefore, the cavity length L of the device A with this construction is designed so that the cavity length L is not longer than 1,000 xcexcm.
The inventors hereof examined the current-optical output characteristic for the case where the cavity length L of the device A with the layer structure shown in FIGS. 1 and 2 was adjusted to 800 xcexcm. Thereupon, the characteristic curve of FIG. 3 and the following new knowledge were obtained.
When a driving current (A1) of about 200 mA was injected, as seen from FIG. 3, a first kink (a1) was generated in the optical output, and the existing linear relation between the driving current and the optical output disappeared. If the driving current was further increased to a level (A2) of about 500 mA, a second kink (a2) was generated in the optical output. Thus, in the case of the device A, the two kinks a1 and a2 were generated in the current-optical output characteristic curve as the driving current was increased.
Accordingly, the inventors hereof first closely examined the oscillation spectrum of the device A. The following is a description of the results of the examination.
(1) FIG. 4 shows an oscillation spectrum obtained when the injected current was at about 200 mA.
As seen from this oscillation spectrum, there is a small number of longitudinal modes which oscillate actually in a gain band g. The intensity of a central longitudinal oscillation mode B0 is 5 dB or more higher than those of side modes B1 and B2. As a whole, single longitudinal mode oscillation that is prescribed by the central longitudinal oscillation mode B0 is dominant.
(2) An oscillation spectrum obtained when the first kink (a1) was generated indicates that the central longitudinal oscillation mode B0 jumps to the side mode B1 at a distance of about 0.4 nm therefrom when the gain band shifts to the longer wavelength side as the temperature of the device rises with the increase of the injected current.
The probability of generation of single longitudinal mode oscillation is related to a spontaneous emission factor (xcex2sp) given by
xcex2sp=xcex93xc2x7xcex4xc2x7K/4xcfx802xc2x7n3xc2x7Vxc2x7xcex4xcex,xe2x80x83xe2x80x83(1)
where xcex93 is the confinement coefficient of the active layer, xcex is an oscillation wavelength, K is a factor reflective of the complexity of the electric field for a transverse mode, n is an equivalent refractive index, V is the volume of the active layer, and xcex4xcex is the half width of the spontaneous emission spectrum. It is believed that the smaller the value xcex2sp, the higher the probability of generation of single longitudinal mode oscillation is.
In the case of the device A, therefore, the oscillation wavelength (xcex) is as short as 0.98 xcexcm, so that xcex2sp is lowered in proportion to the fourth power of xcex. Accordingly, the device A can be supposed to be able to cause single longitudinal mode oscillation with high probability.
The following problem will be aroused, however, if a module is constructed in a manner such that the device A that undergoes single longitudinal mode oscillation is connected to an optical fiber. A laser beam generated by single longitudinal mode oscillation has its noise properties lowered under the influence of return light from an end portion of the optical fiber. Further, the oscillation of the laser beam is made unstable by the return light. Accordingly, an optical output fetched from the module and monitor current are rendered unstable.
In order to use the device A as a reliable pumping light source for optical fiber amplifier, therefore, it is necessary to solve the above problem that is attributable to single longitudinal mode oscillation.
The result (2) implies the following situation. In consideration of gain differences caused between the longitudinal modes for single longitudinal mode oscillation for the aforesaid reason, the longitudinal mode hopping occur which causes substantial discontinuous fluctuations of the optical output when the gain band shifts to the longer wavelength side in response to temperature rise. When the injected current almost reaches the level A1, therefore, the current-optical output characteristic loses its linearity, so that the first kink (a1) is generated.
Then, the inventors hereof observed a far field pattern of the device A and obtained the findings shown in FIG. 5.
In FIG. 5, curve C1 represents a transverse oscillation mode for the case where the injected current is lower than A2, and curve C2 represents a transverse oscillation mode for the case where the injected current is near A2 (or where the second kink a2 is generated).
If the injected current increases to A2, as seen from FIG. 5, unit-modal transverse oscillation modes shift horizontally from the center position of the device A (or undergo beam steering). Thus, the direction of emission of the laser beam changes.
In the case where the module is constructed by connecting the optical fiber to the device A, therefore, the optical output fetched through the optical fiber fluctuates when the injected current reaches a value approximate to A2. This is supposed to result in the generation of the second kink (a2) in the current-optical output characteristic curve.
From these investigations, the inventors discovered that the linearity of the current-optical output characteristic curve can be secured by adjusting the cavity length (L) to a value not smaller than 1200 xcexcm. Preferably, the device has a transverse light confinement structure with the transverse refractive index difference of about 1xc3x9710xe2x88x922 for oscillation modes, the reflectance of the low-reflection film on the one end face is 5% or less, and the active layer is formed of one or two quantum well structures.
An object of the present invention is to provide a semiconductor laser device of the gain-waveguide type, capable of oscillating in a longitudinal multi-mode without generating any kinks in a current-optical output characteristic curve even with use of an injected current of 500 mA or more.
Another object of the invention is to provide a novel semiconductor laser device adapted for use as a high-reliability pumping light source for optical fiber amplifier and connected to an optical fiber to form a module, in which a bad influence of return light can be restrained and there is no possibility of beam steering in a far field pattern, so that fluctuations of fetched optical output can be inhibited.
The inventors hereof conducted the following examinations in the process of investigation to achieve the above objects. These examinations will be described first.
(1) First, single longitudinal mode oscillation occurs with high probability in the case of a semiconductor laser device that oscillates in a short-wavelength band of about 0.98 xcexcm. If the injected current increases, the longitudinal mode hopping occurs which causes substantial fluctuations of the optical output. This results in the development of a first kink (a1) in a current-optical output characteristic curve.
It is known that the intervals between the longitudinal modes are proportional to the reciprocal of the cavity length (L) of the device. Therefore, the intervals between the longitudinal modes can be shortened by increasing the cavity length (L) of the device, so that fluctuations of the optical output caused by the jumping of the longitudinal modes can be reduced, supposedly.
(2) Further, a shift of transverse oscillation modes (beam steering) that causes a second kink (a2) is a phenomenon that takes place from the following cause. As the injected current increases, rise of the temperature is accelerated by resistance heating. The refractive index of a region near the active layer is increased by the thermal lens effect, so that the distribution width of light in the horizontal direction is reduced. Accordingly, the carrier density of a light distribution area is lowered by spatial hole burning of carriers, so that the refractive index increases further. In the end, the refractive index distribution in the horizontal direction is disturbed, so that the transverse light confinement effect is lowered.
In order to prevent the generation of the second kink, therefore, it may be advisable to design the device (cavity) so that its resistance heat is small even when high current is injected. To attain this, it is necessary only that the cavity length of the device be increased to lower the resistance of the device.
(3) If the cavity length (L) of the device is increased, in this case, the quantum efficiency lowers inevitably. However, this can be avoided by using a low-reflection surface as the quantum surface of the device.
In consideration of these circumstances, the inventors hereof varied the cavity length (L) of the device A and examined the current-optical output characteristic of the device. Thereupon, the inventors found that the linearity of the current-optical output characteristic curve can be secured by adjusting the cavity length (L) to a value not smaller than the value mentioned later, and developed the semiconductor laser device according to the present invention.
Thus, according to the invention, there is provided a semiconductor laser device comprising: a laminated structure of a semiconductor material including an active layer formed of a quantum well structure; a low-reflection film formed on one end face of the structure; and a high-reflection film formed on the other end face of the structure; and the cavity length of the device being 1,200 xcexcm or more.
Preferably, the device has a transverse light confinement structure with the transverse refractive index difference of about 1xc3x9710xe2x88x922 for oscillation modes, the reflectance of the low-reflection film on the one end face is 5% or less, and the active layer is formed of one or two quantum well structures.